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semop

SYNOPSIS

int semop(int semid, struct sembuf *sops, unsigned nsops);

PARAMETERS

semid: [in] the semaphore set on which to perform the operations.

sops: [in] points to an array of operations to perfrom.

nsops: [in] the number of element in sops.

DESCRIPTION

Perform operations on a semaphore set. The sops parameter a structure sembuf of the form:

short sem_num

which semaphore to operate on (0 is the first).

short sem_op;

the operation.

short sem_flg;

sem_flg may be IPC_NOWAIT for non blocking calls and SEM_UNDO to specify an operation that will be undone on task termination. The individual operations are perfomed if and only if all of them will succeed.

The possible values of cmd are:

• cmd positive: the value of cmd is added to the value of the semaphore. The calling task must have alter privileges on the semaphore set.

• cmd equals to zero: if the value of the semaphore is zero, the call returns. Otherwise, the call block until: the value of the semaphore becomes zero, the semaphore set is removed, or a signal occurs. The calling task must have read privileges on the semaphore set.

• cmd is lower than zero: if the value of the semaphore is greater than the absolute value of cmd, the value of the semaphore is decremented by the value of cmd and the call returns. Otherwise, the call block until: the value of the semaphore becomes greater than the absolute value of cmd at which time the value of the semaphore is decremented by the value of cmd and then the call returns, the semaphore set is removed, or a signal occurs. The calling task must have alter privileges on the semaphore set.

Whenever a call succeed, the sempid member of the semaphore set structure is set to the pid of the current task and the sem_ctime and sem_otime member are set to the current time.

RETURN VALUE

On success, the call returns zero. On error -1 is returned and errno is set to one of the following values:

• E2BIG: there is too many operations requested (the maximum is SEMOPM.
• EFBIG: the semaphore number of some operation is out of range.
• EINVAL: invalid value for semid or nsops.
• ERANGE: semaphore value out of range.
• EACCESS, EAGAIN, EFAULT, EIDRM, EINTR or ENOMEM.

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